Which term refers to something that is harmful or causes ruin?

The term "baneful" specifically describes something that is harmful or that causes ruin. It is derived from "bane," which refers to a cause of great distress or damage. In various contexts, "baneful" can be used to describe anything from toxic substances to negative behaviors that lead to destruction or suffering.

This makes it the ideal choice when looking for a word that embodies the idea of harm or ruin. In contrast, the other terms do not carry this specific connotation of causing injury or destruction. "Antithetical" generally refers to something that is in direct opposition or contrast to something else. "Bombast" denotes extravagant language that is intended to impress but often lacks real meaning, and "asunder" means to be separated or divided, which doesn't imply harm but rather physical separation.